Steam cracking, also referred to as pyrolysis, has long been used to crack various hydrocarbon feedstocks into olefins, preferably light olefins such as ethylene, propylene, and butenes. Conventional steam cracking utilizes a pyrolysis furnace that has two main sections: a convection section and a radiant section. The hydrocarbon feedstock typically enters the convection section of the furnace as a liquid (except for light or low molecular weight feedstocks which enter as a vapor) wherein it is typically heated and vaporized by indirect contact with hot flue gas from the radiant section and by direct contact with steam. The vaporized feedstock and steam mixture is then introduced into the radiant section where the cracking takes place. The resulting products comprising olefins leave the pyrolysis furnace for further downstream processing, including quenching.
Pyrolysis involves heating the feedstock sufficiently to cause thermal decomposition of the larger molecules. The pyrolysis process, however, produces molecules that tend to combine to form high molecular weight materials known as tar. Tar is a high-boiling point, viscous, reactive material that can foul equipment under certain conditions. In general, feedstocks containing higher boiling materials tend to produce greater quantities of tar.
Conventional steam cracking systems have been effective for cracking high-quality feedstock, which contain a large fraction of light volatile hydrocarbons, such as gas oil and naphtha. However, steam cracking economics sometimes favor cracking lower cost feedstocks containing resids such as, by way of non-limiting examples, atmospheric residue, e.g., atmospheric pipestill bottoms, and crude oil. Crude oil and atmospheric residue often contain high molecular weight, non-volatile components with boiling points in excess of 590° C. (1100° F.). The non-volatile components of these feedstocks lay down as coke in the convection section of conventional pyrolysis furnaces. Only very low levels of non-volatile components can be tolerated in the convection section downstream of the point where the lighter components have fully vaporized.
In most commercial naphtha and gas oil crackers, cooling of the effluent from the cracking furnace is normally achieved using a system of transfer line heat exchangers, a primary fractionator, and a water quench tower or indirect condenser. The steam generated in transfer line exchangers can be used to drive large steam turbines which power the major compressors used elsewhere in the ethylene production unit. To obtain high energy-efficiency and power production in the steam turbines, it is necessary to superheat the steam produced in the transfer line exchangers.
Cracking heavier feeds, such as atmospheric and vacuum gas oils, produces large amounts of tar, which leads to coking in the radiant section of the furnace as well as rapid fouling in the transfer line exchangers preferred in lighter liquid cracking service.
Additionally, during transport some naphthas are contaminated with heavy crude oil containing non-volatile components. Conventional pyrolysis furnaces do not have the flexibility to process residues, crudes, or many residue or crude-contaminated gas oils or naphthas which comprise non-volatile components.
To address coking problems, U.S. Pat. No. 3,617,493, which is incorporated herein by reference, discloses the use of an external vaporization drum for the crude oil feed and discloses the use of a first flash to remove naphtha as vapor and a second flash to remove vapors with a boiling point between 230 and 590° C. (450 and 1100° F.). The vapors are cracked in the pyrolysis furnace into olefins and the separated liquids from the two flash tanks are removed, stripped with steam, and used as fuel.
U.S. Pat. 3,718,709, which is incorporated herein by reference, discloses a process to minimize coke deposition. It describes preheating of heavy feedstock inside or outside a pyrolysis furnace to vaporize about 50% of the heavy feedstock with superheated steam and the removal of the residual, separated liquid. The vaporized hydrocarbons, which contain mostly light volatile hydrocarbons, are subjected to cracking.
U.S. Pat. No. 5,190,634, which is incorporated herein by reference, discloses a process for inhibiting coke formation in a furnace by preheating the feedstock in the presence of a small, critical amount of hydrogen in the convection section. The presence of hydrogen in the convection section inhibits the polymerization reaction of the hydrocarbons thereby inhibiting coke formation.
U.S. Pat. No. 5,580,443, which is incorporated herein by reference, discloses a process wherein the feedstock is first preheated and then withdrawn from a preheater in the convection section of the pyrolysis furnace. This preheated feedstock is then mixed with a predetermined amount of steam (the dilution steam) and is then introduced into a gas-liquid separator to separate and remove a required proportion of the non-volatiles as liquid from the separator. The separated vapor from the gas-liquid separator is returned to the pyrolysis furnace for heating and cracking.
Co-pending U.S. application Ser. No. 10/188,461 filed Jul. 3, 2002, Patent Application Publication US 2004/0004022 A1, published Jan. 8, 2004, which is incorporated herein by reference, describes an advantageously controlled process to optimize the cracking of volatile hydrocarbons contained in the heavy hydrocarbon feedstocks and to reduce and avoid coking problems. It provides a method to maintain a relatively constant ratio of vapor to liquid leaving the flash by maintaining a relatively constant temperature of the stream entering the flash. More specifically, the constant temperature of the flash stream is maintained by automatically adjusting the amount of a fluid stream mixed with the heavy hydrocarbon feedstock prior to the flash. The fluid can be water.
Co-pending U.S. patent application Ser. No. 10/851,878, filed May 21, 2004, which is incorporated herein by reference, discloses a process for reducing fouling during cracking of a hydrocarbon feedstock containing resid which comprises: introducing a mixture stream of heated hydrocarbon feedstock mixed with steam to a flash/separation apparatus to form (i) a vapor phase at its dew point which partially cracks causing a temperature decrease which, in the absence of added heat, causes partial condensation of said vapor phase and (ii) a liquid phase. Partial condensation is reduced or eliminated by adding a heated vaporous diluent, e.g., light hydrocarbon or superheated steam, to the flash/separation apparatus to an extent sufficient to at least partially compensate for the temperature decrease and to dilute and superheat the vapor phase, prior to removing the vapor phase as overhead for subsequent cracking and recovery of cracked product. An apparatus for carrying out the process is also provided.
Co-pending U.S. patent application Ser. No. 10/851,494, filed May 21, 2004, which is incorporated herein by reference, discloses a process for cracking hydrocarbon feedstock containing resid comprising: heating the feedstock; mixing the heated feedstock with steam to form a mixture stream; optionally further heating the mixture; flashing the mixture within a flash/separation vessel to form a vapor phase and a liquid phase; partially condensing the vapor phase within the vessel by contacting the vapor phase with a condenser to condense at least some coke precursors within the vapor while providing condensates which add to the liquid phase; removing the vapor phase of reduced coke precursors content as overhead and the liquid phase as bottoms; heating the vapor phase; cracking the heated vapor phase in a radiant section of a pyrolysis furnace to produce an effluent comprising olefins, and quenching the effluent and recovering cracked product therefrom. An apparatus for carrying out the process is also provided.
Co-pending U.S. patent application Ser. No. 10/851,487, filed May 21, 2004, which is incorporated herein by reference, discloses decoking of a process that cracks hydrocarbon feedstock containing resid and coke precursors, wherein steam is added to the feedstock to form a mixture which is thereafter separated into a vapor phase and a liquid phase by flashing in a flash/separation vessel, separating and cracking the vapor phase, and recovering cracked product. Coking of internal surfaces in and proximally downstream of the vessel is removed by interrupting the feed flow, purging the vessel with steam, introducing an air/steam mixture to at least partially combust the coke, and resuming the feed flow when sufficient coke has been removed. An apparatus for carrying out the process is also provided.
Co-pending U.S. patent application Ser. No. 10/851,434, filed May 21, 2004, which is incorporated herein by reference, discloses a highly efficient vapor/liquid separation apparatus for treating a flow of vapor/liquid mixtures of hydrocarbons and steam, which comprises a substantially cylindrical vertical drum having an upper cap section, a middle section comprising a circular wall, a lower cap section, a tangential inlet to introduce hydrocarbon/steam mixtures, an overhead vapor outlet, and a bottom outlet for liquid. The vessel also comprises an annular structure located in the middle section comprising (i) an annular ceiling section extending from the circular wall and (ii) a concentric internal vertical side wall to which the ceiling section extends. The annular structure blocks upward passage of vapor/liquid mixtures along the circular wall beyond the ceiling section, and surrounds an open core having sufficient cross-sectional area to permit vapor velocity low enough to avoid significant entrainment of liquid.
Co-pending U.S. patent application Ser. No. 10/851,546, filed May 21, 2004, which is incorporated herein by reference, discloses an apparatus and process for cracking hydrocarbon feedstock, wherein the temperature of heated effluent directed to a vapor/liquid separator, e.g., flash/separation vessel, whose overhead is subsequently cracked, can be controlled within a range sufficient so the heated effluent is partially liquid, such as from about 260 to about 540° C. (500 to 1000° F.). This permits processing of a variety of feeds containing resid with greatly differing volatilities, e.g., atmospheric resid and crude at higher temperature and dirty liquid condensates, at lower temperatures. The temperature can be lowered as needed by (i) providing one or more additional downstream feed inlets to a convection section, (ii) increasing the ratio of water/steam mixture added to the hydrocarbon feedstock, (iii) using a high pressure boiler feed water economizer to remove heat, (iv) heating high pressure steam to remove heat, (v) bypassing an intermediate portion of the convection section used, e.g., preheat rows of tube banks, and/or (vi) reducing excess oxygen content of the flue gas providing convection heat.
Co-pending U.S. patent application Ser. No. 10/851,486, filed May 21, 2004, which is incorporated herein by reference, discloses a process for cracking hydrocarbon feedstock containing resid comprising: heating the feedstock, mixing the heated feedstock with a fluid and/or a primary dilution steam stream to form a mixture, flashing the mixture to form a vapor phase and a liquid phase which collect as bottoms and removing the liquid phase, separating and cracking the vapor phase, and cooling the product effluent, wherein the bottoms are maintained under conditions to effect at least partial visbreaking. The visbroken bottoms may be steam stripped to recover the visbroken molecules while avoiding entrainment of the bottoms liquid. An apparatus for carrying out the process is also provided.
In using a flash to separate heavy liquid hydrocarbon fractions containing resid from the lighter fractions which can be processed in the pyrolysis furnace, it is important to effect the separation so that most of the non-volatile components will be in the liquid phase. Otherwise, heavy, coke-forming non-volatile components in the vapor are carried into the furnace causing coking problems.
Increasing the cut in the flash/separation vessel, or the fraction of the hydrocarbon that vaporizes, is also extremely desirable because resid-containing liquid hydrocarbon fractions generally have a low value, often less than heavy fuel oil. Vaporizing some of the heavier fractions produces more valuable steam cracker feed. This can be accomplished by increasing the flash/separation vessel temperature to increase the cut. However, the resulting vaporized heavier fractions tend to partially condense in the overhead vapor phase resulting in fouling of the lines and vessels downstream of the flash/separation vessel overhead outlet.
Accordingly, it would be desirable to provide a process for converting materials in the liquid phase in the flash/separation vessel to materials suitable as non-fouling components for the vapor phase.
Co-pending U.S. patent application Ser. No. 10/851,495, filed May 21, 2004, which is incorporated herein by reference, discloses a process and control system for cracking a heavy hydrocarbon feedstock containing non-volatile hydrocarbons comprising heating the heavy hydrocarbon feedstock, mixing the heated heavy hydrocarbon feedstock with a dilution steam stream to form a mixture stream having a vapor phase and a liquid phase, separating the vapor phase from the liquid phase in a separation vessel, and cracking the vapor phase in the furnace, wherein the furnace draft is continuously measured and periodically adjusted to control the temperature of the stream entering the vapor/liquid separator and thus controlling the ratio of vapor to liquid separated in the separation vessel; and wherein in a preferred embodiment the means for adjusting the draft comprises varying the speed of at least one furnace fan, possibly in combination with adjusting the position of the furnace fan damper(s) or the furnace burner damper(s).
Other applications of relevance to the various embodiments of the invention described herein are set forth in co-pending U.S. patent applications, Ser. No. 10/975,703, filed Oct. 28, 2004; Ser. No. 10/891,795, filed Jul. 14, 2004; Ser. No. 10/891,981, filed Jul. 14, 2004; Ser. No. 10/893,716, filed Jul. 16, 2004; Ser. No. 11/009,661, filed Dec. 10, 2004; and Ser. No. 11/068,615, filed Feb. 28, 2005, all of which are incorporated herein by reference.